1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp, having a closed chamber filled with rare gas and a metal halide containing at least Na halide, Sc halide and In halide, an internal volume of the closed chamber being 50 μl or less, and electrodes provided so as to be opposed to each other.
2. Description of the Related Art
FIG. 8 shows a conventional discharge lamp. The discharge lamp has such a structure that a front end portion of a quartz-glass arc tube 5 is supported with one lead support 2 protruded forward from an insulating base 1, a rear end portion of the arc tube 5 is supported with a concave portion 1a of the base 1, and the arc tube 5 is sustained at a portion near its rear end with a metal supporting member 4 fixed to a front surface of the insulating base 1. A front end-side lead wire 8 led from the arc tube 5 is fixed to the lead support 2 by the welding, while a rear end-side lead wire 8 is passed through a bottom wall 1b constituting the concave portion 1a of the base 1 and secured to a terminal 3 provided to the bottom wall 1b by the welding. A symbol G is a cylindrical ultraviolet shielding globe made of the glass to cut off an ultraviolet component in a bandwidth that is harmful to a human body from the light that is emitted from the arc tube 5. This ultraviolet shielding globe G is deposited integrally to the arc tube 5.
Then, the arc tube 5 has such a structure that a closed glass globe 5a in which electrodes 6, 6 are provided between a pair of front and rear pinch sealed portions 5b, 5b to oppose to each other and into which luminous substances i.e., Na halides, Sc halides or Hg, are sealed together with a starting rare gas is formed. A molybdenum foil 7 for connecting the electrode 6 protruded into the closed glass globe 5a and the lead wire 8 led from the pinch sealed portion 5b is sealed in the pinch sealed portion 5b, and thus an air tightness in the pinch sealed portion 5b is maintained.
In this case, this Hg sealed in the closed glass globe 5a is a very useful buffer substance to relieve the damage of the electrode by maintaining a predetermined tube voltage and reducing an amount of collision of the electron to the electrode 6. However, such Hg is an environmentally hazardous material. For this reason, recently the development of the so-called mercury-free arc tube into which Hg acting as the environmentally hazardous material is not sealed is accelerated.
It has been proposed in Japanese Patent Unexamined Publications No. JP-A-11-86795 paragraph 0026 and No. JP-A-11-307048 paragraphs 0031 through 0034 that In halide is charged, instead of Hg, in order to maintain a tube voltage.
That is, in JP-A-11-86795, In halide, instead of Hg, is charged by 1 to 100 μmol/cm3 as a voltage gradient formation medium. In JP-A-11-307048, InI is charged in addition to ScI3 and NaI as metal halides, whereby a decrease in the voltage due to mercury free is improved, and a luminescent color, which is within a chromaticity standard range required as a white light source, is obtained.
However, in a development process of a mercury free arc tube, when the inventor charges an amount of InI as disclosed in the JP-A-11-86795 and JP-A-11-307048 into the closed glass globe, the arc tube emits in purplish red color at the early time of starting the discharge. Accordingly, there is a possibility that the emission of the arc tube is misidentified or confused with lighting of a red marker lamp such as a tail lamp or a stop lamp, resulting in an unpreferable problem.
Thus, to confirm this cause, the inventor made an experiment for investigating changes in the luminescent color (chromaticity characteristic curve) of the arc tube at the transient time after starting the discharge till reaching the stable discharge, employing each sample of the mercury free arc tube having a different ratio (wt %) of the charging amount of InI to the amount of metal halides (ScI3, NaI, and InI, etc.) charged together with Xe gas into the closed glass globe.
FIGS. 2A, 2B and 2C are graphs when the ratio (wt %) of the charging amount of InI to the total charging amount of metal halides is 0.1%, 1.0% and 2.9%. In any instances, the graph decreases left obliquely downwards to reach the chromaticity minimal value Pmin, and then rises right obliquely upwards to reach a stable position Pa at which the values of chromaticity x, y are within the chromaticity standard ECE R99 of the white light source during the stable discharge (x≧0.345 y≦0.150+0.640x, x≦0.405 y≧0.050+0.750x (hereinafter referred to as “ECE R99”, its range is indicated by symbol A in FIG. 6) with the elapse of time. That is, as the temperature of the arc tube rises, the luminescent color of the arc tube is firstly white that is luminescent color of Xe, then blue that is luminescent color of In and Sc, and finally red that is luminescent color of Na, whereafter the arc tube transfers to a white stable discharge state in which all the luminous substances emit.
Therefore, when an amount of InI is charged as disclosed in the JP-A-11-86795 and JP-A-11-307048, the ratio (wt %) of the charging amount of InI to the total charging amount of metal halides is too large, so that the luminescent color at the transient time up to reaching the stable discharge is strongly purplish red with the low values of chromaticity x and y, whereby there is a fear that the emission of the arc tube is misidentified for or confused with lighting of the red marker lamp.
Thus, the inventor made a visible evaluation test of the chromaticity minimal value Pmin at the transient time, that is for evaluating whether or not the purplish red color is conspicuous by inspecting the luminescence of the arc tube with the naked eye, for each samples (arc tubes) having a different ratio (wt %) of the charging amount of InI to the total charging amount of metal halides of ScI3, NaI and InI. The evaluation result was that the purplish red color is conspicuous when the chromaticity y minimal value is less than 0.29, but is not so conspicuous in a range where the chromaticity y minimal value is from 0.29 to 0.32, and not conspicuous at all, in other words, with no sense of incompatibility, when the chromaticity y minimal value is 0.32 or greater, as shown in FIG. 3. That is, from FIG. 3, it is concluded that the evaluation of whether or not the purplish red is conspicuous in the luminescent color of the arc tube at the transient time can not be made at the chromaticity x minimal value, but can be made at the chromaticity y minimal value.
Also, it has been found that there is a correlation of almost inverse proportion between the ratio (wt %) of the charging amount of InI to the total charging amount of metal halides and the chromaticity y minimal value, as shown in FIG. 4.
Moreover, if the ratio (wt %) of the charging amount of InI to the total charging amount of metal halides is adjusted in a range from 0 to 3.0 wt % in the correlation as shown in FIG. 4, the tube voltage of the arc tube is only varied in a range of 45V±about 5V, as shown in FIG. 5. Thereby, it has been found that as light change (3% at maximum) in the ratio of charging amount of InI has no influence on the tube voltage.
FIG. 6 is a view showing the chromaticity x, y of luminescence during the stable discharge of the samples (arc tubes) having a different ratio (wt %) of the charging amount of InI to the total charging amount of metal halides of ScI3, NaI and InI. When the ratio (wt %) of the charging amount of InI to the total charging amount of metal halides is from 0.1 to 2.9 wt %, the chromaticity y falls within the chromaticity standard “ECE R99” range A required as the white light source. However, when the ratio (wt %) of the charging amount of In halide to the total charging amount of metal halides is 0 wt %, the chromaticity y (=0.396) is out of the chromaticity standard “ECE R99” range A and the color of light is greenish. Hence, the ratio (wt %) of the charging amount of In halide to the total charging amount of metal halides to be charged into the closed chamber is desirably 0.1 wt % or more.